Amorphous selenium (a-Se) has recently been recognized as a promising material for digital X-ray imaging devices for medical and industrial applications. One such prior art device has been described in a paper entitled "A Large Area Solid-State Detector for Radiology Using Amorphous Selenium", Medical Imaging VI: Instrumentation SPIE 1651, pages 134-143 (1992). This article describes a digital X-ray imaging device in which a high voltage is applied to a selenium (Se) plate to obtain high conversion efficiency of X-rays. Depending on the thickness of the Se film, the DC bias voltage may be over several thousand volts. The use of high voltage on the Se film poses serious risks to any semiconductor device connected to the X-ray conversion plate.
One approach to avoiding such risks involves separating the high voltage parts from the signal detection circuits by inserting an insulator between the readout circuit and Se film. X-ray imaging devices comprising an Se film and insulator (electrode/Se/insulator/readout circuit or readout circuit/Se/insulator/electrode) are described in a paper entitled "A Method of Electronic Readout of Electrophotographic and Electroradiographic Images" D. M. Korn et al, Journal of Applied Photographic Engineering vol. 4, no. 4, Fall 1978, and a paper entitled "Laser Readout of Electrostatic Images", by A Zermeno et al, SPIE vol. 173, Application of Optical Instrumentation and Medicine VII, pages 81-87 (1979). Furthermore, U.S. Pat. No. 5,017,989 (R. A. Street), issued to Xerox Corporation, discloses a configuration of electrode/Se/insulator/readout circuits. In this prior art patent, an improvement is identified in the use of a thin film transistor (TFT) circuit which is overlaid on the insulation film to amplify and output the imaging signal.
In all of the prior art devices referred to above, a signal voltage is capacitively coupled to the readout circuit. Thus, the readout circuit is not capable of resetting the potential of the Se film automatically (i.e. non-destructive readout). After readout, the bias voltage-of the Se film must be inverted, in order to make the signal charge reverse, thereby resetting the potential of the Se film and preparing the panel for the next X-ray exposure.
A significant disadvantage of each of the above-referenced prior art devices is that they are substantially incapable of performing real-time acquisition of X-ray images (i.e. incapable of attaining high speed operation to obtain a video rate signal (e.g. 30 frames/second)).
This disadvantage is caused by two reasons which are discussed in greater detail below under the heading "Detailed Description of the Preferred Embodiment and Further Description of the Prior Art".